Oxide superconductive wire, method of manufacturing the same and the products using the same

ABSTRACT

An oxide superconductive wire is provided by, for example, forming an oxide superconductive layer on a tape-type flexible base. A preliminary compressive strain is applied to the oxide superconductive layer in the longitudinal direction. The remaining strain can be provided by using a base having thermal expansion coefficient larger than that of the oxide superconductive layer and by cooling the same after heat treatment, due to contraction of the base. Since the preliminary compressive strain is applied to the oxide superconductive layer, degradation of superconductivity of the oxide superconductive layer can be suppressed even if the oxide superconductive wire is bent in any direction, compared with the wire without such strain. Therefore, the oxide superconductive wire can be coiled, for example, without much degrading the superconductivity.

TECHNICAL FIELD

The present invention relates to oxide superconductive wires, a methodof manufacturing oxide superconductive wires and products using theoxide superconductive wires. More specifically, the present inventionrelates to an improvement for preventing, when an oxide superconductivewire is bent, degradation of superconductivity caused by the bending.

BACKGROUND ART

Metal superconductors, compound superconductors and oxidesuperconductors have been known and various applications thereof havebeen studied. A superconductor has its electrical resistance made zerowhen it is maintained at a temperature not higher than a criticaltemperature. Generation of high magnetic field, high density transfer oflarge current and so on have been tried utilizing this characteristic.

Recently, attention is beginning to center on oxide superconductivematerials, which have higher critical temperature at which thesuperconductive phenomenon occurs. Such superconductive materials can beused for power transmission and distribution, electrical connectionbetween various equipment and elements, AC coils and so on, when theyare turned into longitudinal wire bodies.

Various methods have been known to fabricate wires of oxidesuperconductive materials. In one method, powder of oxidesuperconductive material is filled in a metal pipe and the cross sectionthereof is reduced. In another, a layer of oxide superconductivematerial is formed on a longitudinal base. Gas phase thin filmdeposition such as vapor deposition, sputtering and CVD may be appliedas a method of forming the oxide superconductive layer.

Generally, oxide superconductive materials are weak on strain,especially tensile strain, and when a tensile strain is generated, forexample, superconductivity such as critical temperature and currentdensity is significantly degraded. When a longitudinal oxidesuperconductive wire is bent, a tensile strain is generated in someportion or other inevitably. In order to lengthen the oxidesuperconductor while suppressing generation of strain such as thetensile strain as much as possible, a method of forming a thin oxidesuperconductive layer on a fiber-type or film-type thin or narrowflexible base has been known. By this method, the wire can be bent to behave smaller diameter with the same allowable strain.

However, there is a limit in the above described method, and oxidesuperconductive wires which are stronger against strain have beendesired for practical use.

Therefore, an object of the present invention is to provide oxidesuperconductive wires which are stronger against strains.

Another object of the present invention is to provide a method ofmanufacturing the above described oxide superconductive wires which arestronger against strains.

A further object of the present invention is to provide products usingthe above described oxide superconductive wires.

DISCLOSURE OF THE INVENTION

The inventors of the present invention have found that the oxidesuperconductor is weak on tensile strain but relatively strong againstcompressive strain, and the inventors have attained the presentinvention based on this finding.

The oxide superconductive wire in accordance with the present inventionhas an oxide superconductive layer formed on a longitudinal flexiblebase, wherein the compressive strain is remained in the oxidesuperconductive layer in the longitudinal direction to solve the abovedescribed technical problem.

The base used in the present invention is, typically, a tape-type orfiber-type base.

Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O, Bi-Pb-Sr-Ca-Cu-O, Tl-Ba-Ca-Cu-O,Tl-Pb-Ba-Ca-Cu-O and other materials are used as the oxidesuperconductive materials for forming the oxide superconductive layer inthe present invention. Any of these materials exhibits superconductivityby heat treatment at 400° to 1000° C. In the present invention, theabove mentioned step of heat treatment and a succeeding step of coolingcan be advantageously used to provide preliminary strain of compressionon the oxide superconductive layer.

In the oxide superconductive wire having an oxide superconductive layerformed on a base by painting or gas phase deposition, which may have astabilizing layer and a protective layer formed in addition to the oxidesuperconductive layer, the cross sectional area of the base is thelargest among those of the oxide superconductive layer, the stabilizinglayer and protective layer. Therefore, the strain in the oxidesuperconductive layer is applied by physical action derived fromexpansion or contraction of the base itself, such as thermal expansionof the base or contraction which occurs when the base is heated orcooled. In this manner, the preliminary compressive strain applied tothe oxide superconductive layer is from the base, and such remainingstrain is typically applied by the following to third methods. The firstto third methods will be described with reference to the figures.

FIG. 1 shows the first method. An oxide superconductive layer 1 formedon a base 2 is heat treated and then cooled. At this time, if there is arelation α₁ <α₂ between the thermal expansion coefficient α₁ of theoxide superconductive layer 1 and the thermal expansion coefficient α₂of the base 2, the preliminary compressive strain is applied to theoxide superconductive layer 1 as shown by an arrow 3 during coolingafter heat treatment.

Materials of the base 2 satisfying the above mentioned relation α₁ <α₂comprise zinc, aluminum, indium, silver, tin, lead, aluminum alloy, andcopper alloy.

FIG. 2 shows the second method. A tensile stress is applied as shown byarrows 4a and 4b to the oxide superconductive layer 1 as well as to thebase 2.

Consequently, a tensile strain is applied to the oxide superconductivelayer 1 and the base 2 as shown by arrows 5 and 6. Heat treatment iscarried out in this state. By the heat treatment, only the tensilestrain in the oxide superconductive layer 1 is released. In order toenable release of the strain only in the superconductive layer, amaterial such as yttria stabilized zirconia (YSZ) or alumina must beused as the material of the base 2, whose tensile strain is not releasedunder the heat treatment condition applied to the oxide superconductivelayer. Then the preliminary compressive strain is applied to the oxidesuperconductive layer 1 as shown by an arrow 7 when it is cooled.

FIG. 3 shows the third method. The oxide superconductive layer 1together with the base 2 are bent with the superconductive layerpositioned outside. Consequently, a tensile strain is applied to theoxide superconductive layer 1 as shown by an arrow 8. A relative tensilestrain is applied as shown by an arrow 9 on the outer circumferentialsurface of the base 2, and a relative compressive strain is applied asshown by an arrow 10 in the inner side. Then heat treatment is carriedout in this state, and only the stress existing in the oxidesuperconductive layer 1 is released. Then the base and thesuperconductive layer are unbent, so that a preliminary compressivestrain is applied as shown by an arrow 11 to the oxide superconductivelayer 1.

A tape-type base 2 may be preferably used when the third method isemployed.

In the third method shown in FIG. 3, the stress existing in the base 2as well may be released during heat treatment.

In the first to third methods described above, the preliminarycompressive strain is applied from the base 2 to the oxidesuperconductive layer 1. However, if a stabilizing layer is formed onthe oxide superconductive layer 1, the preliminary compressive strainmay be applied from the stabilizing layer to the oxide superconductivelayer. This will be described with reference to FIG. 4.

Referring to FIG. 4, an oxide superconductive layer 13 is formed on abase 12, and a stabilizing layer 14 is formed on the oxidesuperconductive layer 13. When the stabilizing layer 14 is formed,heating at a temperature of, for example, 400° C. to 1000° C. is carriedout. At this time, if there is a relation α13<α14 between the thermalexpansion coefficient α13 of the oxide superconductive layer 13 and thethermal expansion coefficient α14 of the stabilizing layer 14, apreliminary compressive strain is applied at least on a surface of theoxide superconductive layer 13, based on the contraction of thestabilizing layer 14 during cooling after the heat treatment. Materialsof the stabilizing layer 14 satisfying the relation α13<α14 of thethermal expansion coefficients comprise zinc, aluminum, indium, silver,tin, lead, copper, and aluminum alloy.

In accordance with the above method, the preliminary compressive straincan be applied to the oxide superconductive layer 13 regardless of therelation between the thermal expansion coefficient α13 of the oxidesuperconductive layer 13 and the thermal expansion coefficient α12 ofthe base 12. Therefore, if there is a relation α13>α12, the method ofapplying the compressive strain from the stabilizing layer 14 isespecially effective.

Further, the present invention provides products using the abovedescribed oxide superconductive wires. In the products, the oxidesuperconductive wire having an oxide superconductive layer formed on onesurface of a longitudinal flexible base is bent with the oxidesuperconductive layer positioned outside and a base positioned insideabout the flexural center.

Coils using the oxide superconductive wires, bobbins around which theoxide superconductive wire is wound, cables having the oxidesuperconductive wire wound spirally on the surface of a longitudinalbody, are the examples of the above mentioned products.

In the oxide superconductive wire according to the present invention,the oxide superconductive layer included therein has a preliminarycompressive strain in the longitudinal direction. When such an oxidesuperconductive wire is bent, the preliminary compressive strain iseither released or further increased. When the compressive strain of theoxide superconductive layer is released, the factor affecting thesuperconductivity is reduced, so that degradation of thesuperconductivity generated when the oxide superconductive wire is bentcan be prevented. Even if the compressive strain is further increased,the oxide superconductive layer is relatively strong against compressivestrain compared with the tensile strain, and therefore thesuperconductivity is not very much degraded.

Therefore, the oxide superconductive wire in accordance with the presentinvention can be bent without much degrading the superconductivity whenit is coiled, either in the direction releasing the compressiveremaining strain or in a direction further increasing the compressivestrain. Therefore, treatment of the oxide superconductive wire can befacilitated.

As described above, according to the present invention, oxidesuperconductive wires strong against bending can be provided, which arereadily used for various magnets, coils, cables and the like.

The oxide superconductive wire in accordance with the present inventioncan be coiled after heat treatment. Conversely, if heat treatment is tobe done after coiling, it is necessary that a bobbin or an insulatinglayer must be formed of a material which can withstand heat treatment.For the oxide superconductive wire of the present invention, bobbinsformed of aluminum alloy, FRP or the like may be used, and common enamelinsulation is available.

In the method of manufacturing an oxide superconductive wire inaccordance with the present invention, the preliminary compressivestrain can be easily applied in the longitudinal direction of the oxidesuperconductive layer, advantageously utilizing the steps of heattreatment necessary for forming the oxide superconductive layer and thesucceeding cooling.

In the product using an oxide superconductive wire in accordance withthe present invention, an oxide superconductive wire having an oxidesuperconductive layer formed on one surface of a flexible base with apreliminary compressive strain applied in the longitudinal direction ofthe oxide superconductive layer is employed. Such an oxidesuperconductive wire is bent with the oxide superconductive layerpositioned outside and the base positioned inside about the flexuralcenter. Therefore, a tensile stress, if any, is applied to the oxidesuperconductive layer. However, such a tensile stress is merely torelease the compressive strain which has been applied to the oxidesuperconductive layer, and therefore influence of the tensile strain,which is undesirable to the oxide superconductive layer, can beprevented more or less. Therefore, degradation of superconductivity ofthe oxide superconductive layer included in the oxide superconductivewire can be prevented.

Consequently, in products such as coils and cables and intermediateproducts such as bobbins using the oxide superconductive wires in theabove described manner, the superconductivity of the oxidesuperconductive wires can be fully utilized. For example, in a compactmagnet for investigation, the oxide superconductive wire must be bent tohave a radius of curvature as small as about 2 to 3 cm, for example. Theoxide superconductive wire of the present invention can be applied tosuch usage without problem. In addition, according to the presentinvention, a cable formed by winding an oxide superconductive wirespirally around a surface of a longitudinal body such as a pipe can beprovided. In the case of such a cable, the longitudinal body such as thepipe can give rigid structure, and therefore the oxide superconductivewire is not necessarily be very rigid.

As described above, bobbins on which the oxide superconductive wires aresimply wound are included in the products using the oxidesuperconductive wires according to the present invention. The oxidesuperconductive wire wound around a bobbin is used in a preparatory stepfor the succeeding processing such as enamel painting of the oxidesuperconductive wire, or in the step of forwarding the oxidesuperconductive wire. The condition of winding the oxide superconductivewire, that is, the direction of bending is taken into consideration notonly in the final products such as coils and cables but also in theintermediate products such as bobbins from the following reasons. Morespecifically, if the tensile strain applied to the oxide superconductivelayer exceeds a prescribed magnitude, the superconductivity providedbefore the application of the tensile strain can not be recovered evenif the tensile strain is removed. Therefore, it is important to treatthe intermediate products such that the tensile strain higher than theprescribed magnitude is not applied to the oxide superconductive layer,even if the intermediate product itself is not set in thesuperconductive state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 show methods of applying preliminary compressive strain toan oxide superconductive layer, respectively;

FIG. 4 is a vertical sectional view showing an oxide superconductivewire provided by Embodiment 1 or 5 of the present invention;

FIG. 5 is a cross sectional view of an oxide superconductive wireprovided by Embodiment 2 or 6 of the present invention;

FIG. 6 is a front view showing a bobbin 19 on which an oxidesuperconductive wire 21 is wound in Embodiment 3 of the presentinvention;

FIG. 7 is an enlarged cross sectional view showing the oxidesuperconductive wire 21 wound around a core 20 of the bobbin 19 shown inFIG. 6.

FIG. 8 is a front view showing a portion of a coil 26 provided byEmbodiment 4 of the present invention;

FIG. 9 is a cross sectional view showing in enlargement a portion of anoxide superconductive wire 25 included in the coil 26 shown in FIG. 8;

FIG. 10 is a front view showing a portion of a coil 31 provided byEmbodiment 7 of the present invention; and

FIG. 11 is a cross sectional view showing in enlargement a portion of anoxide superconductive wire 30 included in the coil 31 shown in FIG. 10.

BEST MODES FOR CARRYING OUT THE INVENTION Embodiment 1

Referring to FIG. 4, an oxide superconductive layer 13 is formed on atape-type base 12, and a stabilizing layer 14 of copper is formedthereon.

More specific method of formation of the oxide superconductive layer 13is as follows.

A superconductive layer 13 of Y₁ Ba₂ Cu₃ O₇₋δ having the thickness of 2μm was formed by laser deposition on a tape-type base 12 of YSZ (9% Y₂O₃ added) having the thickness of 50 μm. The conditions of filmformation are as follows.

Target composition: Y₁ Ba₂ Cu₃ O₇₋δ

Base temperature: 720° C.

Laser peak output: 2 J

Laser pulse width: 15 ns

Laser frequency: 10 Hz

O₂ pressure: 0.01 Torr

Then heat treatment at 950° C. in O₂ atmosphere was carried out for 1hour with 0.1% tensile strain applied to the oxide superconductive layer13, like the oxide superconductive layer 1 of FIG. 3. After heattreatment, strain characteristic of the critical current density Jc(77.3K) was measured. According to the result of measurement,degradation by more than 10% of Jc was not exhibited until the oxidesuperconductive wire was bent to the diameter of 40 mm, no matter towhich side the wire was bent. According to detailed examination of thestrain characteristic of the provided oxide superconductive wire,degradation of Jc was smaller than 5% when the oxide superconductivewire was bent to have the diameter of 40 mm with the oxidesuperconductive layer 13 positioned outside and the base 12 positionedinside about the flexural center, while degradation of Jc was 5 to 10%when the wire was bent in the opposite direction.

REFERENCE EXAMPLE 1

Heat treatment under the same condition as in Embodiment 1 was carriedout of the oxide superconductive layer, without applying the tensilestrain. When the oxide superconductive wire provided in this manner wasbent to the diameter of 40 mm with the oxide superconductive layer madeconcave, degradation of Jc was 8%. When it was bent in the oppositedirection, degradation was 90% or more.

Embodiment 2

Referring to FIG. 5, on a central fiber 15 of alumina, an intermediatelayer 16 of MgO for preventing diffusion was formed, then an oxidesuperconductive layer 17 was formed thereon, and a stabilizing layer 18of copper was further formed thereon.

More specifically, Bi₂ O₃, PbO, SrCO₃, CaCO₃, and CuO were weighed tohave the proportion Bi:Pb:Sr:Ca:Cu=1.7:0.4:2:2:3, calcined for 12 hoursat 830° C., and milled to be used as material powder.

The material powder and polyvinyl alcohol were mixed with weightproportion being 1:1. A base prepared by the central fiber 15 of aluminahaving the diameter of 100 μm and an intermediate layer 16 of MgO of 1μm formed thereon was dipped in the above mixture, and it was fired for3 hours at 860° C. A stabilizing layer 18 of copper of 3 μm was formedthereon by vapor deposition to obtain the an oxide superconductive wire.The stabilizing layer 18 may be formed of aluminum.

The wire formed with 0.1% tensile strain applied during firing as shownin FIG. 2 exhibited the degradation of Jc not higher than 10% even whenit was bent to the diameter of 100 mm.

REFERENCE EXAMPLE 2

In Embodiment 2, the wire formed without applying the tensile strainexhibited the degradation of Jc as high as 90% or more when it was bentto the diameter of 200 mm.

Embodiment 3

A film of Y-Ba-Cu-O superconductive material was formed to be 1 μm inthickness by laser deposition on a base (5 mm in width, 0.1 mm inthickness) of silver with platinum deposited to be 0.1 μm thereon as adiffusion preventing layer. The conditions of film formation were asfollows.

Target composition: Y₁ Ba₂ Cu₃ O_(x)

Film forming temperature: 750° C.

Gas pressure: 0.1 Torr

Gas O₂

Laser wave length: 193 nm

Energy density: 1 J/cm²

After the film is formed in this manner, the provided oxidesuperconductive wire was heat treated for 10 minutes at 900° C. in aheat treatment chamber, with the speed of movement of the base being 4cm/hour.

Then, in a winding chamber, the oxide superconductive wire 21 was woundfor 5 turns on a core 20 of a bobbin 19 as shown in FIGS. 6 and 7. Atthis time, the oxide superconductive wire 21 was wound with the oxidesuperconductive layer 23 positioned outside and the base 24 positionedinside about the center of the core 20, that is, the flexural center 22,as shown in FIG. 7. The diameter of the core 20 was 40 mm.

By the time of the above described step of winding, the heat-treatedoxide superconductive wire 21 was cooled, and since the thermalexpansion coefficient of the base 24 was larger than that of the oxidesuperconductive layer 23, a preliminary compressive strain was appliedin the longitudinal direction of the oxide superconductive layer 23.

The oxide superconductive wire 21 wound around the bobbin 19 was dippedin liquid nitrogen and the critical current was measured, which was 5.5A.

When the oxide superconductive wire provided by Embodiment 3 was cut inan appropriate length without winding, and the critical current inliquid nitrogen of the linear wire was measured, which was 6.0 A.

REFERENCE EXAMPLE 3

The oxide superconductive wire 21 was wound around the core 20 of thebobbin 19 under the same condition as in Embodiment 3 except that theoxide superconductive layer 23 was positioned inside. The criticalcurrent of the oxide superconductive wire 21 wound around the bobbin 19was measured under the same condition, which was 4.75 A.

Embodiment 4

An oxide superconductive wire was provided by using the same base andthe same film forming conditions as in Embodiment 3. By using the oxidesuperconductive wire, a superconductive coil was fabricated as will bedescribed below.

Referring to FIG. 8, an oxide superconductive wire 25 was wound for 5layers to provide a coil 26, starting from a circumference distant fromthe center 27 of the coil 26 by the radius of 20 mm. At this time, theoxide superconductive wire was wound with the oxide superconductivelayer 28 positioned outside and the base 29 positioned inside.

The coil 26 provided in this manner was dipped in liquid nitrogen, andthe critical current was measured, which was 25 A.

REFERENCE EXAMPLE 4

A coil was fabricated under the same condition as in Embodiment 4 exceptthat the wire was wound with the oxide superconductive layer 28positioned inside. The critical current was measured under the samecondition, which value was 8.8 A.

Embodiment 5

Referring to FIG. 4, an oxide superconductive layer 13 was formed on abase 12 of silver having the thickness of 50 μm and the width of 5 mm,and a stabilizing layer 14 was formed thereon. The specific method offorming the oxide superconductive layer 13 and the stabilizing layer 14was as follows.

The oxide superconductive layer 13 of Y₁ Ba₂ Cu₃ O_(x) of 1 μm inthickness was formed by laser deposition on the base 12. The conditionsof film formation were as follows.

Target composition: Y₁ Ba₂ Cu₃ O_(x)

Base temperature: 600° C.

Laser peak output: 1 J

Laser pulse width: 10 ns

Laser frequency: 10 Hz

Gas pressure: 0.1 Torr

Then, the base 12 on which the oxide superconductive layer 13 was formedwas moved to a separate film forming chamber and cooled to 400° C. Inthis state, silver was deposited to the thickness of 0.2 μm by laserdeposition to form a stabilizing layer 14.

The oxide superconductive wire provided in this manner was taken out ofthe film forming chamber, and strain dependency of the critical currentdensity in the liquid nitrogen was measured. When the oxidesuperconductive wire provided in this manner was bent to have thediameter of 40 mm with the oxide superconductive layer 13 positionedoutside of the base 12, the degradation of Jc was smaller than 3%.

REFERENCE EXAMPLE 5

Reference Example 5 is in the scope of the present invention. Thisexample is to confirm the effect of Embodiment 5 described above.

More specifically, an oxide superconductive wire was formed under thesame condition as in Embodiment 5 except that the stabilizing layer wasnot provided, and the strain dependency of Jc was measured. When thewire was bent to the diameter of 40 mm with the oxide superconductivelayer positioned outside of the base, degradation of Jc was smaller than5%.

Embodiment 6

Referring to FIG. 5, on a central fiber of alumina, an intermediatelayer 16 of MgO for preventing diffusion was formed, then an oxidesuperconductive layer 17 was formed thereon, and further a stabilizinglayer 18 of silver was formed thereon.

More specifically, a mixture of oxides having the composition ofBi:Pb:Sr:Ca:Cu=1.8:0.4:2:2:3 was fired and milled to be used as amaterial powder.

The material powder and polyvinyl alcohol were mixed with the weightproportion being 1:1. A base having a central fiber 15 of alumina havingthe diameter of 100 μm with an intermediate layer 16 of MgO of 1 μm inthickness formed thereon was dipped in the above mixture and fired for 3hours at 860° C. Then, while the resulting body was heated to 400° C., astabilizing layer 18 of silver having the thickness of 3 μm was formedby vapor deposition to provide an oxide superconductive wire.

The strain dependency of the critical current density in liquid nitrogenof the provided oxide superconductive wire was measured. When the oxidesuperconductive wire was bent to have the diameter of 100 mm, thedegradation of Jc was not more than 30%, compared with the wire whichwas not bent.

REFERENCE EXAMPLE 6

In Embodiment 6 described above, when the wire without the stabilizinglayer was bent to the diameter of 200 mm, degradation of Jc was 90% ormore.

From the comparison between Embodiment 6 and Reference Example 6, it isunderstood that the preliminary compressive strain has significanteffect, even if it is applied only to the surface layer of the oxidesuperconductive layer 13 as in Embodiment 6.

Embodiment 7

An oxide superconductive wire having a stabilizing layer of silverformed thereon was provided by using the same base under the sameconditions of film formation as in Embodiment 5. The oxidesuperconductive wire was once wound around a bobbin, and then asuperconductive coil was fabricated in the following manner.

Referring to FIG. 10, the oxide superconductive wire 30 was wound to 5layers to provide a coil 31 starting from a circumference distant fromthe center 32 of the coil 31 by the radius of 20 mm. At this time, theoxide superconductive wire 30 was wound with the oxide superconductivelayer 33 positioned outside of the base 34 and the stabilizing layer 35positioned on the outermost side, as shown in FIG. 11.

The coil 31 provided in this manner was dipped in liquid nitrogen andthe critical current was measured, which was 29.2 A.

Further, the experiment was repeated for 50 times in liquid nitrogen andin room temperature, to find stability against heat cycle. The criticalcurrent value after 50 measurements was 28.8 A.

REFERENCE EXAMPLE 7

Reference Example 7 is in the scope of the present invention. Thisexample is to confirm the effect of Embodiment 7.

A coil was fabricated under the same condition as in Embodiment 7 exceptthat the silver stabilizing layer was not provided, and the criticalcurrent was measured under the same condition, which value was 25 A. Inaddition, stability against heat cycle between the liquid nitrogen andthe room temperature was examined, and the critical current value afterthe repetition of the heat cycle for 50 times was 20 A.

INDUSTRIAL APPLICABILITY

As described above, the oxide superconductive wire in accordance withthe present invention can be advantageously applied to products such assuperconductive magnets for investigation, magnets for nuclear magneticresonance diagnostic apparatus, superconductive cables, superconductivegenerators, superconductive transformers, superconductive coils forlinear motor cars, superconductive coils for electromagneticallypropelled ships, and to intermediate products such as thesuperconductive wires simply wound around bobbins.

We claim:
 1. A product using an oxide superconductive tape or wire, saidtape or wire comprising(a) a longitudinal flexible base, and (b) anoxide superconductive layer formed on one surface of the base, saidsurface being an outer surface upon bending, said oxide superconductivelayer having a preliminary compressive strain in the longitudinaldirection.
 2. A product using an oxide superconductive tape or wireaccording to claim 1, wherein said product is a coil using said oxidesuperconductive tape or wire.
 3. A product using an oxidesuperconductive tape or wire according to claim 1, wherein said productis a bobbin around which said oxide superconductive tape or wire iswound.